What is a correct way of sampling affect on the test results?

Avoid external and internal contamination whilst collecting the sample. Take proper precaution to prevent contamination. Mix and rinse thoroughly to obtain an accurate and uniform sample. Adhere meticiously to recommended sample procedure.

use the spectrum analyzer for getting the exact  specifieddata 

There are two possible answers given by the question, first, accurate sampling will give the correct answer, secondly, excess or lack of reagents needed in a certain procedure will not give correct answer. analysing the procedure should always follow the procedure and process the test efficiently as needed.